The present invention relates to the making of rolls or rollers to be used as withdrawal and support rollers for ingots as emerging from a mold for continuously casting; and more particularly, the invention relates to the thermal treatment of such rollers during their completion.
Rollers of the type to which the invention refers are usually provided with a sprayed-on and sintered coating made, e.g. of a nickel-chronium-alloy. As the roller is coated, sintering of the coating requires heating to temperatures from 800.degree. C to 1000.degree. C, and it is inevitable that the roller body be heated, at least in parts, to that temperature. If the rollers have not been heat treated previously to obtain particular toughness and/or tensile strength, the heating during the sintering poses no problems. However, if the roller was particularly normalized and tempered, sintering may cause the roller to loose again the particular mechanical properties. Consequently, one has usually used for the construction of the roller bodies those kinds of materials which do not require such a thermal treatment, that is to say, one uses rollers made out of steel which has a high tensile strength even without strength and toughness enhencing thermal treatment. These are usually highly alloyed steels and very expensive.
Materials that require tempering to be strong at high temperatures are known, for example, under the DIN designation 17 Mn Mo V 64; or 10 Cr Mo 910; 13 Cr Mo 44; 15 Mo 3 or 35 Cr Mo 4. Rolls made of 17 Mn Mo V 64 steel are, for example, thermally treated as follows: The roller is normalized at 920.degree. C for a duration of 2 hours, followed by cooling in the furnace. Next, the roller is tempered at 620.degree. C to 680.degree. C for 3 hours, followed by cooling in air. Now, if the roller is spray coated and sintered at 800.degree. C to 1000.degree. C, the prior normalizing and tempering comes to naught. Moreover, the sinter process may lead to heat cracks in the interior of the roller.